1. Field of the Invention
The present invention relates to compositions which are useful as slurries for the chemical-mechanical polishing of substrates, especially substrates comprised of silica and silicon nitride. More specifically the slurries of the present invention include an aqueous medium, abrasive particles, a viscosity modifier, a surfactant, and a compound which complexes with silica and silicon nitride.
2. Description of Related Art
In integrated circuit technology, various active and passive elements generally must be isolated from one another within the integrated circuit structure. This has often been accomplished by deep or shallow trench isolation techniques. These techniques typically use silicon dioxide (silica) as a dielectric material and silicon nitride as a stop layer, requiring chemical-mechanical polishing (planarization) of each circuit layer. To achieve efficient polishing and adequate planarization, a polishing slurry is generally useful and should provide a high selectivity involving the removal rate of silica relative to silicon nitride.
In Silvestri et al., U. S. Pat. No. 4,526,631, a slurry of 6 weight percent colloidal silica adjusted to a pH of about 12 with KOH provides a polishing ratio of about 10 SiO.sub.2 to 1 Si.sub.3 N.sub.4. Beyer et al., U.S. Pat. No. 4,671,851, states that the polishing ratios between SiO.sub.2 and Si.sub.3 N.sub.4 preferably should be between a lower limit of 4 to 1 and a higher limit of 40 to 1. Beyer describes obtaining a ratio of 6.2 to 1 using a colloidal silica in water with small amounts of a sodium salt of dichloroisocyanuric acid and sodium carbonate.
Even more recent patents such as Murase, U.S. Pat. No. 5,502,007, also describe obtaining selectivities of about 10 SiO.sub.2 to 1 Si.sub.3 N.sub.4 removal rates using a colloidal silica slurry as a polishing agent. Kodera et al., U. S. Pat. No. 5,445,996, use ceria as well as silica for the abrasive particles in slurries, but they also report selectivities for SiO.sub.2 to Si.sub.3 N.sub.4 removal rates in the range of 2 to 3.